A final decision on a replacement for Britain's Trident nuclear deterrent is expected to be delayed until after the next general election, allowing the Tories and Liberal Democrats to take different positions on the renewal during the campaign.

Amid criticism that Nick Clegg is failing to differentiate his party from David Cameron's Conservatives, a senior minister said a delay would undermine Labour attacks on the Lib Dems as "crypto-Tories".

Nick Harvey, the armed forces minister, indicated today that Cameron and Clegg are prepared for an election battle over nuclear weapons when he indicated that the final "main gate" decision would be delayed until October 2015. This irreversible decision is due to be taken in December 2014 or January 2015.

Harvey told the Lib Dem conference in Liverpool this morning: "If it were to be delayed until just after a May 2015 election that is of no great financial significance, it is of no great military significance, it is of no great industrial significance. But, believe me, it is of profound political significance. The Conservatives know that they are not going to be able to look to the Liberal Democrats to help them get that through parliament, so the issue will be a hot potato for Labour."

The minister added that the delay would allow the Lib Dems to neutralise a key Labour attack in the general election and to differentiate Clegg from Cameron. Harvey said: "On the eve of the next general election, determined to portray the Liberal Democrats as crypto-Tories who propped them up for five years, Labour will have the headache of having to ride to the help of the Tories on Trident. The Liberal Democrats are not going to change their mind. As for Labour, watch this space. This story ain't over yet. It is going to run and run."

Liam Fox, the defence secretary, is unlikely to be troubled by a relatively short delay in the "main gate" decision until after the election, though he may quibble with waiting until October 2015. The Guardian reported on Sunday that Fox sees no problem in delaying the Trident announcement by four to six months. Fox's main concern is that the Treasury would like to impose a delay of five years.

The announcement by Harvey means that the Lib Dems and the Tories will put contrasting proposals before the country in the 2015 general election. Fox is pressing hard for a like-for-like replacement of Trident's "continuous at sea deterrent". The system has four Vanguard submarines, one of which is always on patrol, able to launch a strike against any target at any time.

The Lib Dems support a British nuclear deterrent. But they advocate a more modest system based on the Astute class of submarines. This is not a continuous deterrent because the nuclear warheads and missiles are stored away from the Astute submarines which are only launched if Britain faces a clear threat.

Harvey supported an emergency motion at the Lib Dem conference that called for a full review of alternatives to a like-for-like replacement of Trident. The motion added that it is "untenable" for the Trident replacement not to be included in the strategic defence and security review.

The Lib Dem leadership said the motion was consistent with their coalition agreement with the Tories. This said that a replacement for Trident would be subject to a "value for money" assessment and the Lib Dems would have the right to look at "alternatives".

An "initial gate" decision, which will allow for early preparatory work to be undertaken on the Trident replacement, will be made in January. This will cost £700m during the next spending period between 2011-14.

Lady Williams of Crosby, one of the founding members of the SDP, led the charge against a like-for-like replacement of Trident. Williams dubbed Trident a "cold war weapon" and said the arch-Atlanticist Liam Fox should note how the world has moved on.

"Mr Fox, have you noticed that the American people did elect a new president and that Mr [George] Bush is no longer the president of the United States?" she said to applause. "President [Barack] Obama is doing everything in his power to move multilateral disarmament forward … That ratification hangs by a thread. Would it not be ludicrous if the UK moved in the direction of like-for-like replacement to last for the next 40 years, locking our children and grandchildren into the position?"